呼叫小部件時添加特定的驗證器

我試圖呼叫這個包含FormField它的驗證器作為變數的小部件。

    import 'package:flutter/material.dart';

class InputField extends StatelessWidget {
  final String label, label2;
  final String content, content2;
  var fieldController = TextEditingController();
  void fieldValidator =() {
    
  };

  InputField(
      {this.label,
      this.content,
      this.label2,
      this.content2,
      this.fieldValidator
      });

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        return Row(
          children: <Widget>[
            Expanded(
              flex: 1,
              child: Container(
                width: 50.0,
                child: Text(
                  "$label",
                  textAlign: TextAlign.left,
                  style: TextStyle(
                    fontWeight: FontWeight.w900,
                    color: Color.fromARGB(255, 255, 255, 255),
                  ),
                ),
              ),
            ),
            SizedBox(
              width: 5.0,
            ),
            Expanded(
              flex: 3,
              child: Container(
                width: MediaQuery.of(context).size.width / 3.7,
                color: Color.fromARGB(255, 255, 255, 255),
                child: TextFormField(
                  controller: fieldController,
                  validator: fieldValidator,
                  style: TextStyle(
                    fontSize: 15.0,
                    color: Colors.black,
                  ),
                  decoration: InputDecoration(
                    contentPadding: EdgeInsets.all(10.0),
                    hintText: "$content",
                    hintStyle: TextStyle(
                        color: Color.fromARGB(255, 190, 190, 190),
                        fontSize: 14),
                    fillColor: Color.fromARGB(255, 0, 0, 0),
                  ),
                ),
              ),
            ),
          ],
        );
      },
    );
  }
}

我在小部件構造器中設定驗證器，并且每次呼叫小部件時我都想提供自己的驗證器功能：

InputField( label: "City",
           content: "Please provide us your city name",
         fieldValidator:(value) {
                            if (value == null || value.isEmpty) {
                              return 'This one is required';
                            }
                            return null;
                          },),

我在創建小部件的第一頁上收到此錯誤：

    This expression has a type of 'void' so its value can't be used.
Try checking to see if you're using the correct API; there might be a function or call that returns void you didn't expect. Also check type parameters and variables which might also be void.

uj5u.com熱心網友回復：

驗證器應該來自型別FormFieldValidator<String>?而不是，void所以您的驗證器應該如下所示：

FormFieldValidator<String>? fieldValidator =(fieldValue) { 
  return "result";
  };

uj5u.com熱心網友回復：

如果您查看TextFormField，您會看到它validator是這樣定義的，一個可以接收字串并回傳另一個字串的函式。

String? Function(String?)? validator

我建議您為您的 做同樣的fieldValidator事情，以便您的InputField定義如下所示：

class InputField extends StatelessWidget {
  final String label, label2;
  final String content, content2;
  final String Function(String?)? fieldValidator;

  var fieldController = TextEditingController();
...

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